1. Field of the Invention
The present invention relates to a bioreactor for improved productivity of photosynthetic algae. More particularly, the invention is concerned with bioreactors and bioreactor systems for optical enhancement of photosynthetic productivity of algae.
2. Discussion of the Background
In the past decade, there has been considerable activity relating to production of photosynthetic microalgae for commercial purposes. Special industries aiming to produce health food, food additives, animal feed, biofertilizers and an assortment of natural products (most notably .beta.-carotene) have been established. Recently, microalgae have been suggested as a means to sequester carbon from the industry, and hydrogen producing algae as a source of energy.
It is known that algae productivity is limited by three major factors: light, nutrients and temperature. Historically, most efforts have been invested in developing the optimum nutrients for any specific algae. This included means to saturate the photosynthetic system with CO.sub.2. Still, temperature remains a major limiting factor in commercial, outdoor production. Optimal conditions for efficient production are usually selected in accordance with the climatic conditions prevailing in a chosen site. Yet, even in such sites, winter and night temperatures, as well as morning hour temperatures pose serious limitations to growth rates.
Under optimal temperature and nutrient conditions, the single most important factor in limiting productivity is light. In order to address this problem, open algae producing ponds are made as shallow as practically possible for improving the surface to volume ratio, and stirring is increased to reduce the time the algae stay in darkness, which, in concentrated cultures, is a few millimeters from the surface.
Attempts have also been made to grow algae in tubes, again increasing the surface to volume ratio and hence light conditions thereof. Conventional tubes, however, do not allow temperature control and the light is limited by ambient conditions. If light intensity is less than maximum and strong light attenuation with depth of culture occurs, limitations to photosynthetic activity are imposed.
Still further development has recently been made by growing algae in tubes in which special light guides (fiber optics) are inserted in the tubes. These fibers uniformly diffuse a large proportion of the light passing through them. A solar concentrator is used to concentrate solar light onto one end of the fibers. The fiber optics in this arrangement occupy a large proportion of the reactor's volume and a large proportion of the light is lost through the end of the fibers. Since the reactor itself is in the dark, the use of the solar concentrator results in very low light intensities per surface unit within the growth chamber. No temperature control is available in this system.